The lifetime of traditional light sources, such as, for example incandescent, fluorescent, and high-intensity discharge lamps, are estimated through industry-standard lamp rating procedures. Typically, a large, statistically significant sample of lamps is operated until about 50% have failed, which at that point, in terms of operating hours, defines a “rated life” for that lamp. Decrease in lumen output may occur as a lamp or light fixture operates over a long enough period of time. Based on years of experience with traditional light sources, lighting experts often use lamp life ratings, along with known lumen depreciation curves, to design the lighting for a desired space, and to determine re-lamping schedules and economic payback. This aspect of predictive life or half-life of a light source is not particularly true with light emitting diodes (LEDs), which can continue to operate at very low light levels and are at less risk of critical failure compared to traditional light sources. An LED's end-of-life may be measured in terms of lumen flux depreciation to a particular level as defined by manufacturers or by standard test methods.
According to the Alliance for Solid-State Illumination Systems and Technologies (ASSIST), the threshold at which human eyes may detect light output reduction is about 70% lumen maintenance. Further research conducted by ASSIST shows that a 30% reduction in light output may be acceptable to a majority of those who use luminaires (otherwise known as lighting devices or lighting fixtures and is defined as a complete electric light unit, which includes any sort of light source—incandescent, fluorescent, high-intensity discharge lamps, LEDs, and the like) in general lighting applications, however, ASSIST recommends considering both higher and lower figures for lumen maintenance for certain types of applications. These applications may include, for example, a wall-washing application where lights may be seen by users side-by-side, which may require that the useful life of the lumen to be calculated on a higher figure, such as, typically 80% for lumen maintenance. In some applications where light outputs are not critical, such as, for example, decorative light systems, lower lumen maintenance thresholds may be acceptable. ASSIST has proposed that two coordinates be used to express the useful lifetime of an LED component, L70 and L50. Rated lumen-maintenance life is measured in hours with associated percentage of light output, noted as Lp. In other words, L70 of 30,000 hours means that the tested LEDs produce 70% of the initial light output at 30,000 hours. If an LED has L50 of 30,000 hours, its lumen output decays faster than one with L70 of 30,000 hours. Since the temperature of the device may impact these figures, thermal design of devices is critical, but may not be enough. Dimming level and average power of the lighting device, e.g., level of usage, may dictate its L70 and L50. Changes in current may also impact the L50 of the LED device. The same may be true for constant current, where temperature variations will impact the L50 number.
With reference to FIGS. 15 and 16 Philips published a Technology White Paper, entitled Understanding Power LED Lifetime Analysis, date unknown, in which typical graphs of factors impacting LED lifetime were provided. According to FIG. 15, if temperature is constant, changes in current will impact the L50 of the LED device. The same is true for constant current, (see FIG. 16, in this case 1.5 A), where temperature variations will impact the L50 number. Both graphs were taken from the above Philips White paper.
LEDs usually do not fail abruptly like traditional light sources; instead, their light output slowly diminishes over time. Furthermore, LED light sources can have such long lives that life testing and acquiring real application data on long-term reliability becomes problematic—new versions of products are available before current ones can be fully tested. To add more to the challenge, LED light output and useful life are highly dependent on electrical and thermal conditions that are determined by the luminaire and system design. This conclusion is reflected in the Philips Technology White Paper, titled “Understanding power LED lifetime analysis,” which may be found at www.lrc.edu/programs/solidstate/assist/index.asp.
A disadvantage with current LED systems and methods is that there is currently no standard format for reporting LED lifetimes and/or lumen depreciation curves. According to a Paper published by the U.S. Department of Energy, titled “LED Luminaire Reliability,” and retrievable from http://ephesuslighting.com/wp-content/uploads/2014/01/Fact-Sheet-LED-Luminaire-Reliability.pdf, “A test procedure currently in development by the Illuminating Engineering Society of North America (designated LM-80, IESNA Approved Method for Measuring Lumen Maintenance of LED Light Sources) will provide a common procedure for making lumen maintenance measurements at the LED device, array, or module levels”. Further, the U.S. Department of Energy paper states: “The LM-80 test procedure addresses only one factor in the life of an LED luminaire—lumen depreciation of the LED device over the prescribed test period.” For LED light sources, LM-80 defines lumen-maintenance life as “the elapsed operating time at which the specified percentage of the lumen depreciation or lumen maintenance is reached, expressed in hours.” Different from rated life, the rated lumen-maintenance life is defined as “the elapsed operating time over which an LED light source will maintain the percentage (p) of its initial light output.” A disadvantage with this method is that there are usually many additional factors to consider when LEDs are installed in a luminaire or systems that can impact the rate of lumen depreciation or the likelihood of catastrophic failure, and this test method fails to address those additional factors. These additional factors may include temperature extremes, humidity, moisture incursion, voltage or current fluctuations, failure of the driver or other electrical components, damage or degradation of the encapsulation material covering the LEDs, damage to the wire bonds that connect the LEDs to the fixture, and degradation of phosphors.
Current lumen depreciation models may be applied solely to the light source, (i.e., LED Chip) and not the luminaire. The light source is driven at a specified current at three temperatures: 55 degrees Celsius, 85 degrees Celsius and a third manufacturer specified temperature. The light source may be driven at 100% output for a minimum of 6,000 hours. From the data collected, predictions can be made regarding the end of life of the light source. The lifetime predictions of the light source may be inherited by the luminaire that has the tested light source installed. That prediction may fail to take into account additional factors that may affect the projection. These factors may include manufacturing defects, such as, heat sinking/thermal management, ambient temperature of installation, and even control strategy of luminaire in operation—all of which can have a positive and negative impact on the lumen depreciation. LM-82-12, the approved method for the “Characterization of LED Light Engines and LED Lamps for Electrical and Photometric Properties as a Function of Temperature,” addresses/covers the testing of light engines/luminaires and integrated lamps. While the LM-82-12 may cover these additional factors, it may be cost prohibitive for luminaire manufacturers, thus, LM-80 predictions verified to run at a certain temperature within a luminaire may be the norm. A further disadvantage with LM-82 testing, is that even if LM-82 testing has been carried out, it is done at set conditions and cannot compensate for different dimming levels, i.e., wattages and temperatures over time.
Given the state of the art, it is clear that there is a need for a solution that allows users/customers to be able to predict when sufficient lumen degradation has occurred such that action needs to be taken to replace the light source, unless worse case assumptions on running conditions and time are accepted as truth. Customers who may need to install large quantities of luminaires, such as, for example, tens of thousands of luminaire systems, need to have a way to predict when and how much each luminaire has degraded over time. The absence of clear and specific guidance leads to luminaires that may be replaced ahead of time, being replaced too late, and/or having increased/higher costs of maintenance.
Further, there is a need for a system and a method that identifies in real time the current state of a luminaire and notifies a maintenance team when to replace a luminaire, while providing data to the manufacturers of the luminaire about the current state, environmental readings and statistical behavior of their devices installed in customer sites. As used herein, reference to “current” or “real time” means without intentional delay, given the processing limitations of the system and the time required to accurately measure the data. Manufacturers of the luminaires could benefit greatly by receiving real time data related to actual in situ usage of their devices. Control strategy may play an integral role in the lifetime of an LED product, and when a lighting product is installed, a maintenance factor may have been applied to the lighting design to compensate for lumen depreciation. If no control strategy or commissioning has been implemented, this may result in the luminaire running at ˜15% higher output than is necessary, decreasing its useful life. There is therefore a need for a system and method that provides a control strategy that helps facilitate maintenance of a luminaire based on its current state.
Further, there is a need for a method of projecting lumen depreciation over time in a manner that takes into consideration all of the above-mentioned factors. A solution is needed in which technology local to the luminaire is enabled, as well as by the utilization of an Internet Of Things (IoT) network, to provide an accurate solution that is cost effective to both the customers who use the luminaires, as well as the manufacturers of the luminaires.